Apparatus for generating ice nuclei smoke particles for weather modification

ABSTRACT

Ice-nuclei smoke particles are produced through contact of an organic ice-nuclei compound with superheated steam under pressure, followed by internal adiabatic and isentropic expansion using a supersonic nozzle.

United States Patent Fukuta et a1.

APPARATUS FOR GENERATING ICE NUCLEI SMOKE PARTICLES FOR WEATHERMODIFICATION Inventors: Norikiko Fukuta, Seoul, South Korea; Young I-I.Paik, Englewood, Colo.

Assignee: The United States of America as represented by the Secretaryof the Interior, Washington, DC.

Filed: Apr. 16, 1974 Appl. No.1 461,339

Related U.S. Application Data Division of Ser. No. 285,995, Sept. 5,1972, Pat. No. 3,835,059.

[56] References Cited UNITED STATES PATENTS 1,358,084 11/1920 Leigh239/14 3,126,155 3/1964 Lohse 239/14 3,606,971 9/1971 Harrison et a].239/14 3,788,543

1/1974 St. Amand et a1 239/14 Primary ExaminerLloyd L. King Attorney,Agent, or FirmGersten Sadowsky; Donald R. Fraser [57] ABSTRACTIce-nuclei smoke particles are produced through contact of an organicice-nuclei compound with superheated steam under pressure, followed byinternal adiabatic and isentropic expansion using a supersonic nozzle.

U.S. Cl. 239/14 Int. Cl. A01G 15/00; E0111 13/00 Field of Search 239/14,102; 252/305, 359; 4 Claims, 3 Drawing Figures STEAM V 11 3 23PATENTEDAUBI2I975 3.899.129

O ROUND NOZ ZLE LOG NO. OF SMOKE PARTICLES PER UNIT VOLUME o FLAT NOZZLEi MACH NUMBER FIG.3

APPARATUS FOR GENERATING ICE NUCLEI SMOKE PARTICLES FOR WEATHERMODIFICATION This application is a division of application Ser. No.285,995 filed Sept. 5, 1972, now US. Pat. No. 3,835,059.

BACKGROUND OF THE INVENTION A number of different methods are practicedin seeding clouds and fogs. One conventional method of seeding involvesdispersing a fine smoke, spray or dust of a solid substrate (ice-nucleicompound) into a cloud. Currently, silver iodide is the most widelyemployed ice-nuclei compound. In practice, an ice-nuclei compound isevaporated in the flame of a sprayed organic solution and subsequentlyis condensed by contact with cooler outside air. Ice nucleation occursby the collision of water droplets present in a cloud or fog withicenuclei compound particles'introduced therein. lce nucleation takesplace in a cloud as the ice-nuclei compound particles apparently causethe water molecules within a cloud or fog to assume an ice latticestructure. This process requires the temperature of the cloud to bebelow the freezing point of water (C).

Another widely practiced seeding method is evaporating an ice-nucleicompound in a pyrotechnical reaction and then quenching the resultingvapor with cooler outside air.

Another seeding method involves spraying a silver iodide liquid ammoniasolution into the atmosphere.

A fourth method of the prior art involves evaporating an ice-nucleicompound of relatively high vapor pressure in a hot gas (such as the hotexhaust gas of an aircraft engine) and quenching the vapor with theoutside air.

These seeding methods of the prior art generally involve the use of jetor subsonic mixing of the ice-nuclei compound vapor with cooler outsideair. Jet mixing methods involve expelling ice-nuclei compounds (in vaporform) through an orifice directly into the outside air without priorcooling or quenching at velocitiesbelow Mach 1.

In each of the above methods a number of problems arise. The firstmethod necessitates the burning of a combustible, while the second andthird require the handling of inflammable, toxic, or corrosivesolutions. Perhaps the greatest disadvantage to these methods is thatupon quenching each yields low net supersaturation of the ice-nucleicompound vapor. The reason for this low saturation is twofold. First, inorder to reach the supersaturation necessary for particle nucleation,quenching of the ice-nuclei compound vapor is achieved by mixing withoutside air; however, this simultaneously dilutes the vapor and reducesthe supersaturation. Secondly, because of the relatively slow coolingprocess of the prior art, the nucleated particles (which are likely tobe in the liquid state at least at the beginning) abstract the vapor andeither reduce the supersaturation or prevent the level from rising. Aknown disadvantage to using organic ice-nuclei compounds for weathermodification is the relatively low number of active nuclei produced perunit weight of organic material, compared with that of silver iodide.

A known method for the production of organic icenuclei smoke particlesis a vapor mixing technique. This method involves bringing a hot gas incontact with an organic material, having a suitable vapor pressure, toform an organic material, having a suitable vapor pressure, to form anorganic vapor laden mixture. This method suffers from the disadvantagesnoted above in producing a high number of smoke particles; namely, thesimultaneous reduction of supersaturation from diluting the vapor withcooler air occurs, as well as the abstraction of vapor by the growingsmoke particles.

It can be shown theoretically that the rates of nucleation for smokeparticle formation are directly related to the true supersaturation orundercooling of the system. Therefore, creating an effectiveundercooling or means for quenching is essential to generate a sizeablenumber of ice-nuclei smoke particles. The creation of an effective meansof quenching as well as the elimination of the disadvantages of theprior art noted above are accomplished by the process and apparatus ofour invention.

SUMMARY OF THE INVENTION Our method for generating ice-nuclei smokeparticles employs a vapor mixing technique whereby a superheated inertcarrier gas is contacted, under pressure, with a suitable ice-nucleicompound to form a vapor laden gas (i.e., mixture of inert carrier gasand icenuclei compound vapor). The vapor laden gas is then quenchedinternally by nearly adiabatic and isentropic expansion. The vapor ladengas after quenching may be dispersed into a cloud or fog in the form ofice-nuclei smoke particles. These ice-nuclei smoke particles nucleatewith the water in a cloud, under proper temperature conditions, andresult in the formation of ice crystals.

Practically, the process involves the production of superheated steamwhich is introduced into a chamber containing a suitable ice-nucleicompound to form a vapor laden steam of the compound vapor and steam.The vapor laden steam is then quenched by means of a supersonic nozzle,resulting in the formation of icenuclei smoke particles suitable forweather modification.

The supersonic nozzle is employed to overcome the disadvantages ofquenching with external cold air. A supersonic nozzle is known toperform nearly adiabatic and isentropic expansion. The quenching orundercooling is only a function of the Mach number or speed of thenozzle at constant temperature.

Our invention provides for the internal cooling of vapor laden gas(i.e., prior to contact with outside cold air). As a result, our processsharply raises the supersaturation level, at least at the stage ofparticle nucleation. Furthermore, the rapid cooling rate of ourinvention reduces the vapor abstraction effect of particles alreadyformed, allowing more particles to nucleate. Cloud seeders may find ourinvention useful in combination with cloud seeding agents which do notproduce a detrimental effect upon the environment. This advantage isparticularly important when one considers the possible detrimentalinfluence to the ecology of silver from the current weather modificationagent, silver iodide.

DESCRIPTION OF THE PREFERRED EMBODIMENTS A way of determining theeffectiveness of ice-nuclei smoke particle formation is to determine thesaturation ratio of the particles in air. The saturation ratio is aratio of the true vapor pressure to the saturated vapor pressure. In thecase of nominal saturation ratios, such values are strictly theoreticalcalculations, whereas actual saturation ratios are based on observedvapor pressures.

The nominal saturation ratio created in supersonic flow is of the orderof 10 for typical operating conditions, while that for jet mixing is ofthe order of 10 under the same conditions. The nominal level ofsupersaturation cannot be reached in the supersonic nozzle because ofthe vapor abstraction by the particles formed; However, tests clearlyshow that the supersonic condensation method increases the smokepartic'le concentration by a factor of 10 compared with that in the jetmixing method.

A supersonic nozzle consists of converging and diverging sections. Thenozzle may be flat or round. Its dimensions are simply determined by thespeed or Mach number of the nozzle (i.e., the velocity at which a gaspasses through the nozzle) and the nozzle flow capacity. Nozzle speedsgreater than Mach 1 have proven effective for increasing particleconcentration and saturation ratios. A wide range of nozzle capacitiesmay be employed as the value is not critical. The capacity should besufficient to generate the desired number of particles within aparticular time. Capacities as low as one liter/second are effective,yet greater capacities are useful. For practical cloud seedingoperations, the nozzle 'should be designed to give a speed of Mach 2.0to 2.5.

A chemically inert carrier gas is needed to generate ice-nuclei compoundvapor. Ice-nuclei compounds, such as organics, tend to decompose whenstrongly heated during evaporation into the carrier gas. Steam, heatedwell above the saturation temperature, is a suitable carrier gas.Furthermore the high heat content, nontoxic nature, safety andavailability of steam make it particularly advantageous.

The ice-nuclei compound must be capable of forming an ice latticestructure upon nucleation. A number of materials, both inorganic as wellas organic are suitable, but the organic are preferred in our invention.Phloroglucinol, 1-5 dihydroxynaphthalene, and metaldehyde are well knownas organic ice-nuclei compounds. While the steam pressure of thegenerator system is determined by the supersonic nozzle employed, thesteam temperature depends upon the ice-nuclei compound used. Thetemperature of the evaporator chamber must not exceed the thermaldecomposition temperature of the ice-nuclei compound. The icenucleicompound is generally in powder form, and cannot be gaseous or liquid atambient conditions. The following table lists some suitable ice-nucleicompounds and the temperature range under which they may be evaporated.

Ice-Nuclei Compound Steam temperature Metaldehyde 120C 160CPhloroglucinol l50C 2l9C I70C 265C l-5 dihydroxynaphthalene BRIEFDESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic of the ice-nucleismoke particle generator assemblage.

FIG. 2 is a schematic of the nozzle adapter.

FIG. 3 is a graphic comparison of smoke particle concentration withvarying nozzle speed.

The generator assemblage 10, consists of four major units: a steamgenerator 11, an evaporator 12 for icenuclei compounds such as organics,a supersonic nozzle system 13, and a heat source 14. The steam generatorl1 and heat source 14 serve to produce the superheated steam necessaryfor the process.

Water is introduced into the steam generator through an inlet 19, whichmay be opened and closed as needed by a suitable valve 20. The water isadded to the desired level, as indicated by awater level gauge 15, andthen heated by the desired heat source to produce steam. The pressure ismonitored by a suitable gauge 18. A bleeder valve 16 may be provided toeliminate unused steam, and a safety valve 17 is provided to allow forthe elimination of steam when the system reaches undesired pressures.

A heat source, such as electric coils, or a solid flame burner 22, isprovided below the generator. Propane, natural gas, or city gas aresuitable fuels.

Steam produced in the generator 11 rises under pressure upward throughthe main steam line to a junction 23 where a portion flows from the mainline to the gas flow control 24 (a pressure sensitive valve, whichadjusts the gas flow 33 to the burner 22') while the remaining steam isintroduced via a superheating coil 25 into the evaporator 12, whichcontains the ice nuclei compound 27, through small nozzles 26 directedonto the surface of the compound. The vapor ladensteam which forms thenpasses back into the line 28 and is further heated either bysuperheating coils 29 oralternatively by a heat exchanger 30, in orderto prevent the deposition of compound and water vapor in the nozzlesystem 13. The vaporladen steam then is discharged via a nozzle 31.

FIG. 2 specifically illustrates passage of the 'v apor laden steam afterleaving the evaporator and passing through an adapter (heat exchanger)30 which also allows the preheating and cleaning of the nozzle by simplypassing a hot steam source 32 outside as controlled by a three-way valve35. The vapor laden steam passes through the line, after beingsuperheated, to the converging diverging supersonic nozzle 31.

The temperature of evaporator is controlled so as not to exceed thethermal decomposition of the ice-nuclei compound.

The flow within the system of hot and dry'steam as well as the vaporladen steam is clearly indicated by the arrows in FIGS. 1 and 2.

The following examples are provided to more fully illustrate theinvention but are not to be construed as limiting the scope thereof. 9

EXAMPLE 1 16 liters of deionized water were added to a steam generatoras illustrated in FIG. 1. A liquid propane gas cylinder having acapacity of 5 gallons provided the solid flame heat. The safety valvewas designed to have a cracking pressure of 150 psi. Steam formed in thegenerator was introduced through a A inch I.D. copper tubing via asuperheating coil into an evaporator (shatter proof and having a 1 gal.capacity) containing 2 Kg. of metaldehyde. The temperature of theevaporator was maintained at 140C. A supersonic nozzle having a capacityof 2 liters/sec. and a speed of Mach No. 2.1 was employed. The steampressure was maintained at 107.4 psi. The ice-nuclei smoke particlesgenerated were found acceptable for seeding a cloud having a temperatureof -2C or below. Injection of the metaldehyde smoke particles producedin clouds of higher temperature yield no immediate effect. However, uponsubsequent cooling nucleation occurs.

EXAMPLE 2 In the same manner as Example 1, 2 Kg. of l-5dihydroxynaphthalene was tested. The evaporator temperature wasmaintained at 200C and a suitable smoke resulted.

EXAMPLE 3 Ice-nuclei smoke particles produced in the manner of Examples1 and 2 were immediately diluted by introduction into one end of adilution tunnel. The tunnel consisted of a steel tube 62 cm in diameter,330 cm long, and a variable speed electric fan placed in the middle ofthe tube creating a wind tunnel effect. The smoke was transferred to anultramicroscope cell by means of a syringe which collected the resultingdiluted air at the opposite end of the tunnel to determine the particleconcentration. The concentration (number per unit colume) was readilydeduced from the magnification and dilution factors by taking asuccession of 30 snap counts and averaging. The average radius of l-5dihydroxynaphthalene smoke particles was less than 0.1 pm, while thatfor metaldehyde was approximately 0.2 pm. FIG. 3 graphically illustratesthe results of determining the concentration of l-5 dihydroxynaphthalenesmokes at varying nozzle Mach numbers. At Mach 1, the concentration inundiluted expanded state was 2.1 X particles cm. The highestconcentration in undiluted expanded state observed was 1.5 X 10particles cm at Mach 2.3. No difference in concentration was observed inusing flat and round nozzles having the same Mach number.

Thus it is clear that the supersonic condensation method increases thesmoke particle concentration by a factor of 10 at the most, over that ofthe sonic mixing process.

The invention in its broader aspects is not limited to the specificdetails shown and described, but departures may be made from suchdetails within the scope of the accompanying claims without departingfrom the principles of the invention.

We claim:

1. In an apparatus for generating ice-nuclei smoke particles comprising:

means for generating superheated steam,

an enclosed containing means holding therein an ice nuclei compound,

a multi-opening fluid discharge means maintained within said containingmeans where multi-openings of said discharge means are disposed todirect fluid discharged therefrom at the surface of said compound,

a conduit means connected to an outlet of said generating means and toan inlet of said discharge means whereby said conduit means carriestherein said superheated steam from said generator to said dischargemeans wherefrom steam discharged at said compound surface evaporatescompound into said steam,

at converging-diverging nozzle means,

a further conduit means having connections to an outlet of saidcontaining means and to an inlet of said nozzle means and constituting apassage for steam laden with vapors of said compound leaving saidcontaining means and entering said nozzle, wherein said vapor ladensteam is quenched substantially by adiabatic and isentropic expansion atsupersonic velocities.

2. In the apparatus of claim 1 wherein said further conduit comprises acoil disposed for heating by a heat source whereby said vapor ladensteam in passing Mach 2.5 and a capacity of about 2 liters per second.=l l=

1. In an apparatus for generating ice-nuclei smoke particles comprising:means for generating superheated steam, an enclosed containing meansholding therein an ice nuclei compound, a multi-opening fluid dischargemeans maintained within said containing means where multi-openings ofsaid discharge means are disposed to direct fluid discharged therefromat the surface of said compound, a conduit means connected to an outletof said generating means and to an inlet of said discharge means wherebysaid conduit means carries therein said superheated steam from saidgenerator to said discharge means wherefrom steam discharged at saidcompound surface evaporates compound into said steam, aconverging-diverging nozzle means, a further conduit means havingconnections to an outlet of said containing means and to an inlet ofsaid nozzle means and constituting a passage for steam laden with vaporsof said compound leaving said containing means and entering said nozzle,wherein said vapor laden steam is quenched substantially by adiabaticand isentropic expansion at supersonic velocities.
 2. In the apparatusof claim 1 wherein said further conduit comprises a coil disposed forheating by a heat source whereby said vapor laden steam in passingtherethrough is further heated.
 3. In the apparatus of claim 1 whereinsaid nozzle means discharges said vapor laden steam at a speed greaterthan Mach
 1. 4. In the apparatus of claim 1 wherein said nozzle meanshas a speed of from about Mach 2.0 to about Mach 2.5 and a capacity ofabout 2 liters per second.